Operating System Supports virtual machines o Promises each process - - PowerPoint PPT Presentation

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Processes

CS 217

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Operating System

• Supports virtual machines
• Promises each process the illusion of

having whole machine to itself

• Provides services:
• Protection
• Scheduling
• Memory management
• File systems
• Synchronization
• etc.

Hardware OS Kernel User Process User Process

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What is a Process?

• A process is a running program with its own …
• Processor state

– EIP, EFLAGS, registers

• Address space (memory)

– Text, bss, data, heap, stack

Hardware OS Kernel User Process User Process

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Operating System

• Resource allocation
• Sharing
• Protection
• Fairness
• Higher-level abstractions
• Common strategies
• Chop up resources into small pieces and

allocate small pieces at fine-grain level

• Introduce level of indirection and

provide mapping from virtual resources to physical ones

• Use past history to predict future behavior

Hardware OS Kernel User Process User Process

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Life Cycle of a Process

• Running: instructions are being executed
• Waiting: waiting for some event (e.g., i/o finish)
• Ready: ready to be assigned to a processor

Create Ready Running Termination Waiting

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Context Switch

Running Running Save context Load context Save context Load context

. . . . . .

Running Waiting Waiting Waiting

Process A Process B

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Overlap CPU with I/O operations

• Schedule CPU for process B

while process A is waiting for I/O

• Better utilize CPU

CPU CPU CPU I/O I/O I/O

A:

CPU CPU CPU I/O I/O I/O

B:

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Process Control Block

• For each process, the kernel keeps track of ...
• Process state (new, ready, waiting, halted)
• CPU registers (EIP, EFLAGS, EAX, EBX, …)
• CPU scheduling information (priority, queues, ...)
• Memory management information (page tables, ...)
• Accounting information (time limits, group ID, ...)
• I/O status information (open files, I/O requests, ...)

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Fork

• Create a new process (system call)
• child process inherits state from parent process
• parent and child have separate copies of that state
• parent and child share access to any open files

pid = fork(); if (pid != 0) { /* in parent */ ... } /* in child */ ... Parent Child

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Wait

• Parent waits for a child (system call)
• blocks until a child terminates
• returns pid of the child process
• returns –1 if no children exist (already exited)
• status

#include <sys/types.h> #include <sys/wait.h> pid_t wait(int *status);

• Parent waits for a specific child to terminate

#include <sys/types.h> #include <sys/wait.h> pid_t waitpid(pid_t pid, int *status, int options);

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Fork

• Inherited:
• user and group IDs
• environment
• close-on-exec flag
• signal handling settings
• supplementary group IDs
• set-user-ID mode bit
• set-group-ID mode bit
• profiling on/off/mode status
• debugger tracing status
• nice value
• stdin
• scheduler class
• all shared memory segments
• all mapped files
• file pointers
• non-degrading priority
• process group ID
• session ID
• current working directory
• root directory
• file mode creation mask
• resource limits
• controlling terminal
• all machine register states
• control register(s)
• Separate in child
• process ID
• address space (memory)
• file descriptors
• active process group ID.
• parent process ID
• process locks, file locks, page locks,

text locks and data locks

• pending signals
• timer signal reset times
• share mask

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Exec

• Overlay current process image with a specified image file

(system call)

• affects process memory and registers
• has no affect on file table
• Example:

execlp(“ls”, “ls”, “-l”, NULL); fprintf(stderr, “exec failed\n”); exit(1);

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Exec (cont)

• Many variations of exec

int execlp(const char *file, const char *arg, ...) int execl(const char *path, const char *arg, ...) int execv(const char *path, char * const argv[]) int execle(const char *path, const char *arg, ..., char * const envp[])

• Also execve and execvp

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Fork/Exec

• Commonly used together by the shell

... parse command line ... pid = fork() if (pid == -1) fprintf(stderr, “fork failed\n”); else if (pid == 0) { /* in child */ execvp(file, argv); fprintf(stderr, “exec failed\n”); } else { /* in parent */ pid = wait(&status); } ... return to top of loop ...

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System

• Convenient way to invoke fork/exec/wait
• Forks new process
• Execs command
• Waits until it is complete

int system(const char *cmd);

• Example:

int main() { system(“echo Hello world”); }

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Summary

• Operating systems manage resources
• Divide up resources (e.g., quantum time slides)
• Allocate them (e.g., process scheduling)
• A processes is a running program with its own …
• Processor state
• Address space (memory)
• Create and manage processes with ...
• fork
• exec
• wait
• system }

Used in shell